1. Field of the Invention
The present invention pertains to radio frequency devices and, more particularly, to a patch antenna configuration for use on a conductive surface of an object to be monitored.
2. Description of the Related Art
Communication using a passive (non-powered) device where an interrogation signal is modulated and reflected by the passive device is known. One example of this form of communication is in the radio frequency identification (RFID) and auto identification technologies, which use backscatter communication.
Backscatter communications involves selectively changing and reflecting a received signal. For example, modulating the radar cross-section of a target causes energy reflected off the target to contain the information in its phase or amplitude modulation. A unit receiving the reflected energy, typically called a “reader,” which usually has supplied the original energy required for this communication, is configured to extract the data in the signal by comparing the received reflected signal to the original interrogation signal.
By way of analogy, a reader in the form of a flashlight has its beam of light aimed at a person with a mirror. The individual with the mirror begins selectively reflecting the flashlight's beam back to the flashlight. The selective reflection can be done in a digital fashion, i.e., off and on. This allows the person holding the mirror to communicate without the necessity of generating additional energy.
In RFID technology, commercial backscatter communications systems utilize microwave frequencies. This technology, which has been available for several decades, uses a target, called a “tag,” to respond to an interrogation signal for identification purposes.
More particularly, and by way of example, referring to FIG. 1, shown therein is a basic RFID system 10 that includes three components, an antenna 12, a transceiver with decoder 14, and a transponder or tag 16 having its own antenna 18. In operation, the transceiver 14 generates electro-magnetic radio signals 22 that are emitted by the antenna 12 and are received by the tag 16. When the tag 16 is activated by the signal, data can be read from or written to the tag 16.
In certain applications, the transceiver antenna 12 is a component of the transceiver and decoder 14, which makes it an interrogator 20 (or reader), and which can be configured either as a hand-held or fixed-mount device. The interrogator 20 emits the radio signals 22 in a range from one inch to 100 feet or more, depending upon its power input and the radio frequency used. When the RF tag 16 passes through the radio signals 22, the tag 16 detects the signal 22 and is activated. Data encoded in the tag 16 is then transmitted through reflection by a modulated data signal 24 through the antenna 18 in the tag 16 and to the interrogator 20 for subsequent processing.
RFID tags that are powered by the interrogation signal are often referred to as a passive device because they derive the energy needed for operation from the radio frequency energy beamed at it. The tag rectifies the field and dynamically changes the reflective characteristics of the tag antenna, creating a change in reflectivity that is seen at the interrogator. In contrast, a battery-powered semi-passive RFID tag operates in a like fashion, modulating its RF cross-section in order to reflect a delta to the interrogator to develop a communication link. Here, the battery is the source of the tag's operational power. In an active RFID tag, a transmitter is used to create its own radio frequency energy powered by the battery.
Conventional continuous wave backscatter RF tag systems that utilize passive (no battery) RF tags require adequate power from the signal 22 to power the tag's internal circuitry used to modulate the signal back to the interrogator 20. Efficient collection of this energy from the signal 22 is necessary to maximize system performance. Impedance matching of antenna circuit components at the desired frequency is one method to optimize efficiency. However, size and performance constraints of RFID tag systems render existing impedance matching designs infeasible. Another disadvantage is the restrictions imposed on signal power and data flow in the RF signals by government regulation.
Known antenna configurations used on radio frequency identification applications include dipole antennas and patch antennas. Microstrip patch antennas are well known because of their relatively thin makeup, making them ideal for use in tagging almost any object.
Patch antennas typically consist of a dielectric substrate having a ground plane formed on one surface and a patch formed on an opposing surface that receives and radiates signals. The size and shape of the patch is designed to accommodate the operating frequency of the circuit to which the patch antenna is coupled, and it is typically designed to provide a suitable radiation resistance.
Because typical radio frequency identification systems operate on high frequencies, the patch antenna can be of a relatively very small size, enhancing its use in applications where weight and size are important. The antenna is associated with a tag or transponder that has a code stored therein, typically in binary format. This code is used by the tag hardware to modulate an interrogation signal and radiate the modulated signal back to the source of the interrogation signal. In other configurations, the transponder antenna receives a signal from a reader and stores data or alters data stored in the transponder, as well as backscatter modulating the received signal back to the reader in order to pass information to the reader. The reader then decodes the signals, as described above, to obtain information from the transponder.
While patch antennas have generally performed well, they suffer from certain limitations. One drawback is the limited range of transmission of the signals from the transponder to the reader. Another difficulty arises from radio frequency interference. Interference can be produced from the transponder's antenna itself or from surrounding objects, including the object on which the transponder is attached. Such interference can prevent the reader from properly detecting the pattern of binary information radiated to it from the tag.
In many cases, attaching a tag antenna to a conductive object can severely degrade the performance of the tag's antenna or completely prevent backscatter modulation of a received signal. Hence, there is a need for an antenna architecture that can be used with conductive objects without increasing weight or size.